1. Field of the Invention
The present invention relates to the field of flat panel displaying, and in particular to a backlight module.
2. The Related Arts
Liquid crystal displays (LCDs) have a variety of advantages, such as thin device body, low power consumption, and being free of radiation, and are thus of wide applications, such as mobile phones, personal digital assistants (PDAs), digital cameras, computer monitors, and notebook computer screens.
Most of the liquid crystal displays that are currently available in the market are backlighting liquid crystal displays, which comprise an enclosure, a liquid crystal panel arranged in the enclosure, and a backlight module mounted in the enclosure. The structure of a conventional liquid crystal panel is composed of a color filter (CF) substrate, a thin-film transistor (TFT) array substrate, and a liquid crystal layer arranged between the two substrates and the principle of operation is that a driving voltage is applied to the two glass substrates to control rotation of the liquid crystal molecules of the liquid crystal layer in order to refract out light emitting from the backlight module for generating images. Since the liquid crystal panel itself does not emit light, light must be provided from the backlight module in order to normally display images. Thus, the backlight module is one of the key components of the liquid crystal displays. The backlight modules can be classified in two types, namely a side-edge backlight module and a direct backlight module, according to the site where light gets incident. The direct backlight module comprises a light source, such as a cold cathode fluorescent lamp (CCFL) or a light-emitting diode (LED), which is arranged at the backside of the liquid crystal panel to form a planar light source directly supplied to the liquid crystal display panel. The side-edge backlight module comprises an LED light bar, serving as a backlight source, which is arranged at an edge of a backplane to be located rearward of one side of the liquid crystal display panel. The LED light bar emits light that enters a light guide plate (LGP) through a light incident face at one side of the light guide plate and is projected out of a light emergence face of the light guide plate, after being reflected and diffused, to pass through an optic film assembly so as to form a planar light source for the liquid crystal display panel.
Referring to FIG. 1, which is a schematic view showing the structure of a conventional backlight module, the backlight module comprises a backplane 20, a plurality of LED substrates 14 arranged in the backplane 20 and comprising LED light sources 24 mounted thereon, a light guide plate 13 arranged in the backplane 20, a reflection plate 17 arranged under the light guide plate 13, a diffusion plate 18 arranged above the light guide plate, an LED driving circuit substrate 15 arranged under the LED substrate 14, and a the light shielding cover 16 arranged in the backplane. The backplane 20 comprises a bottom board 20a and a plurality of side boards 20b connected to the bottom board 20a. The LED substrate 14 comprises a plurality of LED light sources 24 and a tail end section 26. The LED light sources 24 are mounted on a surface of the LED substrate 14 that faces the light guide plate 13. The tail end section 26 is a thin plate and is formed by extending downward from a lower surface of the LED substrate 14. The tail end section 26 comprises an electrically conductive layer connected to corresponding ones of the LED light sources 24. The light guide plate 13 is made of a transparent resin (such as acrylic resin) and comprises a bottom surface 13c facing the bottom board 20a, a top surface 13b distant from the bottom board 20a, and a plurality of side surfaces 13a connected between the bottom surface 13c and the top surface 13b. The LED substrate 14 is arranged parallel to the side surface 13a of the light guide plate 13 and lights emitting from the LED light sources 24 of the LED substrate 14 enters from one of the side surfaces 13a and exits from the top surface 13b that is perpendicular to the side surfaces 13a so as to project the light out. The reflection plate 17 is arranged between the bottom surface 13c of the light guide plate 13 and the bottom board 20a of the backplane 20 to reflect light projecting from the bottom surface 13c back to the light guide plate 13. The diffusion plate 18 is arranged on the top surface 13b of the light guide plate 13 to diffuse and supply the light projecting from the top surface to a liquid crystal panel. Referring to FIG. 2, the LED driving circuit substrate 15 comprises a circuit substrate 28, a driving circuit 29, and receptacles 30. The circuit substrate 28 is an insulation substrate. The circuit substrate 28 has a surface on which the driving circuit 29 that comprises an IC (Integrated Circuit) chip. The circuit substrate 28 has an opposite surface on which a plurality of receptacles 30 is mounted. The driving circuit 29 is arranged parallel to the bottom surface 13c of the light guide plate 13 and positioned against the bottom board 20a of the backplane 20. The driving circuit 29 drives the LED light sources 24 of the LED substrates 14 and is electrically connected to the receptacle 30. The receptacles 30 each comprise therein an insertion hole 31 that is in the form of a thin slit has an inside surface coated with an electrically conductive layer. The tail end section 26 of the respective LED substrate 14 is insertable into the insertion hole 31. Through the tail end section 26 and the insertion hole 31, the LED substrate 14 and driving circuit substrate 15 can be readily combined and separated. The light shielding cover 16 is made of metal and comprises a first light shielding portion 16a and a second light shielding portion 16b, wherein the first light shielding portion 16a is arranged between a side surfaces 13a of the light guide plate 13 and the LED substrate 14 and the second light shielding portion 16b is arranged on the top surface 13b of the light guide plate 13 to block light at the edge of the top surface 13b of the light guide plate 13. The first light shielding portion 16a comprises an opening 16c that is sized corresponding to outside dimensions of the LED light sources 24 to allow light emitting from the LED light sources 24 to pass therethrough and also to retain the LED substrate 14. With a receptacle arranged on driving circuit substrate and a tail end section (functioning like a plug) arranged on the LED substrate, the LED substrate and the driving circuit substrate can be readily combined or separated thereby reducing the steps of assembling the backlight module and achieving simplification of installation and removal of the backlight module.
However, although the LED substrate 14 and the driving circuit substrate 15 are not set to position against the side boards 20b of the backplane 20, in an actual manufacturing process, the LED substrate 14 and the driving circuit substrate 15 are positioned against the side boards 20b of the backplane 20. Since the light shielding cover 16 is made of metal, heat dissipation of the LED substrate 14 is generally done through the light shielding cover 16. However, the light shielding cover 16 is set in tight engagement with the light guide plate 13, the light guide plate 13 is readily heated and gets expanded. Thus, when the LED substrate 14 and the driving circuit substrate 15 are positioned against the side boards 20b of the backplane 20, if the light guide plate 13 is heated and expanded and the light shielding cover 16 does not deform in compliance therewith, the light guide plate 13 would be compressed, leading to ready deformation and cracking.